1. Field of the Invention
The present invention relates to a method of forming a microscopic pattern. More particularly, the present invention is directed to a method in which a microscopic pattern is formed at high precision by using a film which prevents reflection and a resist film in the etching process.
2. Description of the Prior Art
As is well known, in order to manufacture a magnetic bubble memory device and various semiconductor devices, various kinds of very fine patterns such as transfer patterns and interconnection patterns must be formed at high precision.
Such microscopic patterns are usually formed by the technique called "photolithography."
A photoresist film is deposited on a film to be worked (although the object to be worked is not always a film, it is frequently in the shape of a film), for example, a metal film or an insulator film. A desired part of the photoresist film is irradiated with light (ordinarily, ultraviolet rays are employed) to change the solubility of the irradiated portion. When the photoresist film is of the negative type, the irradiated portion has its solubility decreased and is hardened, whereas when the photoresist film is of the positive type, the irradiated part has its solubility increased and is thereby softened.
When the part of the photoresist film having the increased solubility is subsequently removed (this step is usually called "development"), a pattern of the photoresist is formed. Therefore, when the exposed part of, for example, the metal film is removed by etching, a microscopic pattern made of the metal film is formed.
The development of the photoresist film and the removal of the part to-be-removed of the metal film or the like have utilized wet type developing processes and etching processes employing various liquid developers and etchants. Recently, in order to form a still finer pattern at a still higher precision, various dry etchings have been often adopted instead of the wet type developments and etchings.
In conventional photolithography, however, there has developed the following problem. When the resist film is irradiated by light through a mask, the light is reflected by the underlying layer on which the resist film is deposited. For this reason, also a part of the resist film other than the desired part has its solubility changed due to irradiation by the reflected light. Thus, it is difficult to form a microscopic pattern.
In an attempt to prevent such unfavorable influence by the reflecting light, there have been proposed methods wherein a reflection reducing film is interposed between the film to form the microscopic pattern and the photoresist film, whereupon the exposure is carried out. For example, a method wherein a chromium oxide (Cr.sub.2 O.sub.3) film is disposed as the reflection reducing film between a permalloy film and the photoresist film has been proposed. Exposure and development are then conducted, and using the resultant pattern of the resist film as a mask, the permalloy film is etched by ion milling. This procedure has been proposed by Shusaku Matsuyama, et al. in "Fabrication of 3 .mu.m Bubble 80 kbit Chips," Japanese Journal of Applied Physics, vol. 16, (1977), September 16, pp. 351-354.
A method wherein a polycrystalline silicon film is deposited between an aluminum film and the photoresist film, and the pattern of the aluminum film is formed by wet etching has been disclosed in Japanese Patent Application Laid-open Specification No. 49-84788 (1974).
In these prior-art methods, however, the Cr.sub.2 O.sub.3 film and the polycrystalline silicon film employed for the prevention of the reflection are low in immunity against dry etching, so that they are almost useless as a mask in the case of etching the film to form the microscopic pattern. Moreover, the etching of the film to form a microscopic pattern is executed without selectively etching only the reflection reducing film in advance.
For these reasons, dry etching is carried out by employing, in effect, only the pattern of the photoresist as the mask. It is feared that the resist pattern is removed before completion of the step of the dry etching, and it is difficult to precisely form the microscopic pattern.